1. Field of the Invention
The present invention relates to an image reading device having a reading reference position mark to be a reference for reading an image of a recording material such as manuscript paper or a book and an image forming apparatus having this image reading device provided in an apparatus main body.
2. Related Background Art
Up to now, there is known an image reading device for reading an image of a document line by line by an image sensor. Such an image reading device has a home position sensor that uses a photodiode or the like for detecting a reference position of the image sensor.
In recent years, for the purpose of reducing costs, an image reading device having a structure for detecting a reference position of an image sensor without using a photodiode or the like, such as the one as described in Japanese Patent Application Laid-Open No. 2000-113163, has become well known.
As shown in FIG. 11, this conventional image reading device is provided with an image sensor 61, a copy board glass 52 that is a transparent member for mounting a document, a mark 51 consisting of a black area 53 and a white area 54 on the copy board glass 52, and the like.
This image reading device (apparatus) 50 moves the image sensor 61 in a sub-scanning direction (an arrow D direction), detects the black area 53 and the white area 54 by the image sensor 61 and sets a reference position 55 in a boundary between the black area 53 and the white area 54. The image reading device 50 starts reading of a document (original) with a position to which the image sensor 61 moves by a predetermined length L from the reference position 55 as a reading start position 56.
In addition, the image reading device 50 stores the reference position 55 that is the boundary between the black area 53 and the white area 54 in a memory unit. Initialization and movement of the image sensor 51 at the input of a power supply is performed on the basis of the reference position 55. In addition, movement of the image sensor 51 after starting document reading is performed on the basis of the reference position 55 read out from the memory unit.
However, when the image sensor is initialized after inputting a power supply or when data of the reference position stored in the memory unit is destroyed due to some reason, in order to detect the black area, the conventional image reading device moves the image sensor while reading an image line by line to detect the black area and the white area if the image sensor is in a position apart from the black area and the white area. Thus, the image reading device requires a long time for the detection.
In addition, the image reading device may misread the reference position if a document having a pattern similar to the black area and the white area is placed on the copy board glass.